Technical Abstract:
In the Pacific Northwest of the USA, insect control in potatoes is focused on the green peach aphid due to its importance in the transmission of potato leaf roll virus. Colorado potato beetle is controlled incidentally by systemic insecticides, such as aldicarb, directed at the aphid. These systemic insecticides may be unavailable in the near future because of impending regulatory actions, requiring development of control strategies for the beetle. Microbial agents offer safe and selective means of control for several insect pests including CPB with minimal effect on nontarget organisms. The efficacy of Beauveria bassiana and Bacillus thuringiensis as microbial control agents of CPB has been well documented, however, the majority of the research has been conducted in areas where humidity remains fairly high during the growing season. Relatively little research on microbials for control of CPB has been carried out in irrigated desert. In studies conducted in three consecutive seasons we investigated the effect of combining and alternating the B. bassiana and Bt for control of CPB larvae. We also investigated the effects of microbial treatments on feeding by beetle larvae and their effect on yield. Four weekly applications of Bt, B. bassiana, and a mixture of Bt + B. bassiana and alternations of the two microbials (two Bt treatments followed by two of B. bassiana ) were made in experimental plots to determine effects on densities of CPB, defoliation, and yield in 1998-2000. Unsprayed and aldicarb-treated plots were included as experimental controls. For both defoliation and yield in 1998, the mixture of both microbial agents substantially outperformed plots that received Bt alone or B. bassiana alone. Field trials of two microbial control agents (individually and mixed), transgenic potato (Newleaf), and aldicarb for control of CPB and their effects on nontarget organisms were conducted in 1999. Effective control of CPB populations was observed following just two applications of B. thuringiensis or mixtures of Bt and B. bassiana. In 2000 similar levels of control were obtained with Bt and with the mixture of half label rates of Bt and B. bassiana throughout most of the growing season. The treatments had a significant effect on numbers of living overwintering beetles. The mixture of these two pathogens appears to provide an alternative to systemic insecticides for managing CPB in central Washington.